A = [3, 0, 0] ; B = [0, 4, 0] ; C = [0, 0, 2]
a)
X = [3, 0, 0] + r·([0, 4, 0] - [3, 0, 0]) + s·([0, 0, 2] - [3, 0, 0])
b)
X = [3, 0, 1] + r·([0, 4, 0] - [3, 0, 0]) + s·([0, 0, 2] - [3, 0, 0])
c)
X = [3, 0, 0] + r·[0, 0, 1]
d)
([3, 0, 0] + [0, 4, 0] + [0, 0, 2]) / 3 = [1, 4/3, 2/3]